1. Technical Field
The present specification discloses a power converter. In particular, the present specification discloses a power converter including heat sinks to cool a series connection of two switching elements and these switching elements.
2. Description of Related Art
Most of power converters such as bi-directional DC-DC converters and inverters include series connections each of which includes two switching elements. For example, a three-phase AC inverter is provided with a circuit including three sets of series connections connected in parallel, each set including two switching elements. Examples of switching elements may typically include transistors, such as insulated gate bipolar transistors (IGBTs), and metal oxide semiconductor field effect transistors (MOSFETs).
Noise is generated due to ON-OFF operation of switching elements. Voltage fluctuations due to the ON-OFF operation become greater at a middle point of a series connection than at a high-potential terminal and a low-potential terminal thereof. Noise generated due to voltage conversion at the middle point of the series connection may influence other devices.
Common-mode noise is derived from noise generated due to voltage fluctuations at the middle point, propagating through a ground terminal of a system including a power converter, and then returning to the switching elements; and if a propagation path of the common-mode noise is long, the common-mode noise becomes radiation noise, and influences other devices. For convenience of explanation, hereinafter, the expression “common-mode noise” is used for representing noise generated due to voltage fluctuations at the middle point of the series connection, and propagating via the ground.
In the meantime, since switching elements of power converters give off a great deal of heat, many power converters include heat sinks. Conductive plates connected to switching elements have excellent thermal conductivity, and thus the heat sink may be disposed to face conductive plates with an isolating layer interposed therebetween. A material having high thermal conductivity such as copper is used for a heat sink, and such a material is often an electrically conductive material. Hence, the heat sink and the conductive plates disposed with an isolating layer interposed therebetween compose capacitors. Such capacitors are called stray capacitances. By connecting the electric conductive heat sink to the ground terminal, common-mode noise can propagate through a casing of the power converter and the above stray capacitances to return to the switching elements. As a result, the path of the common-mode noise becomes smaller, to thus reduce radiation noise.
Japanese Patent Application Publication No. 2008-294216 further discloses a technique of reducing the common-mode noise. The technique is as follows. Three conductive plates (copper patterns) face a heat sink (a cooling plate) made of copper with an isolating plate interposed therebetween. The heat sink is connected to a ground terminal. A high-potential terminal of a series connection of two switching elements is connected to a first conductive plate, and a low-potential terminal of the series connection is connected to a second conductive plate. A middle point of the series connection is connected to a third conductive plate. The isolating layer has a larger thickness only at a position corresponding to the third conductive plate. Hence, a stray capacitance between the third conductive plate and the heat sink, that is, a stray capacitance between the middle point and the ground becomes smaller. As a result, the common-mode noise transmitted to the ground terminal is suppressed.